US 7930111 B2 Abstract A system for detecting differential bearing damages includes a synthesized tachometer that generates a tachometer signal corresponding to the race speed difference of a bearing assembly such that the bearing damage speed difference dependency can be eliminated and the damage features can be enhanced. The system also includes acceleration enveloping in the cycle domain to further enhance the damage signatures.
Claims(23) 1. A method of detecting differential bearing damages, the method comprising:
providing a synthesized tachometer configured to generate a speed signal for a differential bearing assembly such that the speed signal corresponds to the approximate location of a missing tachometer signal based on race speeds of the differential bearing assembly;
providing a sampling mechanism configured to synchronously sample vibration data associated with the differential bearing assembly based on the speed signal to generate synthesized cycle domain data corresponding to at least one damage signature there from;
causing rotation of the differential bearing assembly and generating a plurality of speed signals for inner and outer rotating bearing races in response to the rotation;
synthesizing a missing tachometer signal corresponding to the speed difference of the inner and outer races via the synthesized tachometer based upon the plurality of speed signals;
synchronously sampling bearing vibration data of the differential bearing assembly via the sampling mechanism in response to the synthesized speed difference tachometer signal to generate synthesized cycle domain data corresponding to at least one bearing damage signature of the differential bearing; and
displaying a spectrum of resultant differential bearing damage signatures in the order domain on a graphic display device in response to the synthesized cycle domain data.
2. The method according to
assuming the existence of a tachometer pulse at a start time;
locating a tachometer pulse at a first time subsequent to the start time and immediately preceding a missing tachometer pulse;
assuming the existence of a tachometer pulse at a second time corresponding to the missing tachometer pulse;
determining an average bearing assembly shaft speed between the first time and the second time; and
minimizing the absolute value of the difference between a deviation in the first time and a deviation in the second time to determine the approximate location of the at least one synthesized tachometer signal.
3. The method according to
4. The method according to
5. The method according to
preconditioning and digitizing the vibration data at a desired high A/D sampling rate;
bandpass filtering the preconditioned and digitized vibration data to isolate signals in a desired frequency range of interest; and
applying the Hilbert transform to the isolated signals to generate an envelope of the isolated signals.
6. The method according to
7. The method according to
8. The method according to
9. A method of enhancing a differential bearing damage signature, the method comprising:
providing a synthesized tachometer configured to generate a speed signal for a differential bearing assembly such that the speed signal corresponds to the approximate location of a missing tachometer signal based on race speeds of the differential bearing assembly;
providing a sampling mechanism configured to synchronously sample vibration data associated with the differential bearing assembly based on the speed signal to generate synthesized cycle domain data corresponding to at least one damage signature there from;
causing rotation of the differential bearing assembly and generating a plurality of speed signals for each differential bearing race in response to the rotation;
synthesizing at least one missing tachometer signal for at least one differential bearing race via the synthesized tachometer based upon the plurality of speed signals;
synchronously sampling vibration data of the differential bearing assembly with respect to race speed differences via the sampling mechanism in response to the plurality of speed signals and the at least one synthesized tachometer signal to generate synthesized cycle domain data corresponding to at least one bearing damage signature; and
displaying a spectrum of resultant differential bearing damage signatures in the order domain on a graphic display device in response to the synthesized cycle domain data.
10. The method according to
11. The method according to
locating a tachometer pulse at a first time subsequent to a start time and immediately preceding a missing tachometer pulse;
assuming the existence of a tachometer pulse at a second time corresponding to the missing tachometer pulse;
determining an average bearing assembly shaft speed between the first time and the second time; and
minimizing the absolute value of the difference between a deviation in the first time and a deviation in the second time to determine the approximate location of the at least one synthesized tachometer signal.
12. The method according to
13. The method according to
preconditioning and digitizing the vibration data at a desired high A/D sampling rate;
bandpass filtering the preconditioned and digitized vibration data to isolate signals in a desired frequency range of interest; and
applying the Hilbert transform to the isolated signals to generate an envelope of the isolated signals.
14. The method according to
15. The method according to
16. The method according to
17. The method according to
18. A system for detecting differential bearing damage, the system comprising:
a synthesized tachometer configured to generate a speed signal for a differential bearing assembly such that the speed signal corresponds to the approximate location of a missing tachometer signal based on both inner and outer race speeds of the bearing assembly;
a sampling mechanism configured to synchronously sample vibration data associated with the differential bearing assembly based on the speed signal to generate synthesized cycle domain data corresponding to at least one differential bearing assembly damage signature; and
a graphic display device configured to display a spectrum of resultant differential bearing damage signatures in the order domain in response to the synthesized cycle domain data.
19. The system for detecting bearing damage according to
20. The system for detecting bearing damage according to
a vibration sensor configured to monitor each bearing race and generate vibration signals there from; and
a signal processing means configured to precondition and digitize the vibration signals to generate the vibration data there from.
21. The system for detecting bearing damage according to
a bandpass filter configured to filter the preconditioned and digitized vibration data to isolate signals in a desired frequency range of interest; and
a signal processing means configured to apply a Hilbert transform to the isolated signals to generate an envelope of the isolated signals corresponding to the vibration data.
22. The system for detecting bearing damage according to
23. The method according to
Description The invention relates generally to engine bearing vibration signatures, and more particularly to a sampling and acceleration enveloping technique for enhancing differential bearing damage signatures associated with engine differential bearings. Differential bearings are some of the most vulnerable parts of an engine and are also some of most difficult parts of an engine for which to monitor the operational condition. Vibration signatures provide the most reliable early warning data associated with regular rolling-element bearing systems. In that regard, the acceleration enveloping based technique has existed for many years. The synchronous sampling technique is also widely used in bearing signature enhancement, especially in variable speed applications. Synchronous sampling is a technique for converting equal time sampling to equal shaft circumferential angle sampling, so that the rotor speed dependency is eliminated. This is usually achieved by installing an encoder on to the bearing which is used to monitor the shaft operation by counting the physical events of the rotating part passing through a stationary detector. Both bearing races in a differential bearing operation however, are in motion, and the race speeds are usually not accurately controlled during bearing operations. Further, the differential bearing assembly is buried under other mechanical components; and the bearing signatures are proportional to the speed difference between the races. Synchronous sampling therefore is required to extract the inherently small and speed difference dependent signatures. Encoders for the differential speed are not physically feasible for a differential bearing due to the moving races. It would be advantageous to provide a sampling technique that overcomes the disadvantages described above associated with traditional sampling techniques for ascertaining differential bearing damage signatures during bearing operations. Briefly, in accordance with one embodiment of the invention, a method of detecting differential bearing damages comprises: generating a plurality of speed signals for inner and outer rotating bearing races; synthesizing a tachometer corresponding to a speed difference of the inner and outer races; and synchronously sampling bearing vibration data in response to the synthesized speed difference tachometer signal to generate synthesized cycle domain data corresponding to at least one bearing damage signature. According to another embodiment of the invention, a method of enhancing a differential bearing damage signature comprises: generating a plurality of speed signals for each differential bearing race; synthesizing at least one tachometer signal for at least one differential bearing race; and synchronously sampling vibration data associated with the differential bearing with respect to race speed differences in response to the plurality of speed signals and the at least one synthesized tachomenter signal to generate synthesized cycle domain data corresponding to at least one bearing damage signature. According to yet another embodiment of the invention, a system for detecting bearing damage comprises: a synthesized tachometer configured to generate a speed signal for a bearing assembly such that the speed signal corresponds to the approximate location of a missing tachometer signal based on race speeds of the bearing assembly; and a sampling mechanism configured to synchronously sample vibration data associated with the bearing assembly based on the speed signal to generate synthesized cycle domain data corresponding to at least one bearing assembly damage signature. These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. A background in acceleration enveloping and synchronous sampling principles is now set forth below with reference to Generally, there are four frequencies associated with a rolling element bearing. These include: 1) Cage frequency or Fundamental Train Frequency (FTF); 2) Rolling element frequency; 3) Ball passing inner raceway frequency; and 4) Ball passing outer raceway frequency. In many industrial applications, the outer raceway
The spin frequency for the rolling element
Acceleration Enveloping or demodulation is a signal processing technique that greatly enhances an analyst's ability to determine the condition of rotating equipment. Briefly speaking, the enveloping technique removes low frequency high amplitude signals and detects low amplitude high frequency components to enhance the damage signature. The isolated higher frequency defect signatures are then converted into frequency domain using rectification and envelope detection. Vibrations occur at multiples and submultiples of the shaft speed for rotating machinery. For example, if the shaft is rotating at 3600 rpm, which is 60 Hz, then responses at multiples of this frequency, sometimes at a fraction of this frequency, can be seen. These multiples are the orders (or harmonics in musical terms). The general relationship between the order ODR, the shaft speed RPM, and the frequency f in Hz is
The purpose of using order instead of frequency Hz, is that the order remains constant with shaft speed; first order is always at the shaft speed; second order is always twice shaft speed, and so on. A sampling technique other than sampling at equal increments of time, such as sampling at equal increments of rotation, must be used for rotating machinery applications. Sampling at equal increments of rotation is called synchronous sampling. The synchronous sampling technique is a very useful for rotating machinery related data processing, especially for those applications with varying shaft speeds. If the Fourier transform is performed on the synchronously sampled data, the result is a set of data in a function of a frequency type scale; but now it is in increments of Orders not Hz. The order analysis can be achieved by conducting a regular FFT and then converting the frequency domain into an order domain, using the shaft speed signal for constant shaft speed cases. If the speed is changing over the length of the FFT, then the order domain amplitude will be smeared over a range of orders. In dealing with signals from rotating machinery, synchronous sampling is preferable, but is difficult in practice. It is impossible to sample synchronously with some data acquisition equipment, in particular those with σ−δ type analog-to-digital converters (ADCs), where it must sample at regular time steps. The present inventors recognized one solution is to use signal processing to digitally resample the data. With the correct signal processing algorithms, the data can be resampled from the initial equi speed time increment data into equi spaced angle increment data, with the help of a once-per-rev tachometer signal from the shaft. A once per revolution signal cannot be physically obtained when both races are moving since the damage frequencies are a function of race speed difference. Keeping the above principles in mind, a synthesized synchronous sampling technique to achieve the desired effective synchronous sampling for such applications is now described below with reference to Equal time sampled data can be easily converted into equal space data using a tachometer, such as shown in - 1) First, assume existence of a tachometer pulse at time zero;
- 2) Once the i
^{th }tachometer pulse is located at time t_{1}, assume the (i+1)^{th }tachometer pulse is located at time t_{2}; - 3) Calculate the average shaft speed, n, from t
_{1 }to t_{2 }and formulate: Δ*t*_{1}*=t*_{2}*−t*_{1 }and Δ*t*_{2}=60/*n;* - 4) Search t
_{2 }such that |Δt_{1}−Δt_{2}| is minimized and such that t_{2 }is then the approximate location of the (i+1)^{th }tachometer pulse; and - 5) Perform synchronous sampling with respect to speed differences once the tachometer signals for each race are obtained.
The vibration sensor data Synchronous sampling A fast Fourier transform (FFT) is applied to the cycle domain data to generate the desired order analysis The outer race of a differential bearing in one application was embedded with an EDM scratch. Based on Eq. (12) the frequency at the speed configuration was determined to be 1850 Hz, or 15.835 order of the speed difference. Without use of synthesized synchronous sampling and acceleration enveloping techniques described above, it was almost impossible to identify any signature from the regular FFT spectrum of an accelerometer signal, as seen in top portion of With regular acceleration enveloping, a small bump around 1850 Hz, as seen in the middle portion of The damage signature was greatly enhanced on a graphic display device such as, without limitation, a CRT of flat panel display, as seen in the bottom portion of The principles described above are suitable for applications such as, without limitation, aircraft engine differential bearing applications in which the engine includes an HP shaft and a LP shaft where both the inner race and outer race are rotating. The principles described above are also suitable for wind turbine applications that employ substantial gearing arrangements and that can generate multiples of harmonics of the fundamental frequency of interest. The embodiments described herein can advantageously be employed using one or more broadband sensors that are disposed on an aircraft engine case, far away from the bearing of interest, to provide an extremely low signal to noise ratio environment. The principles described above advantageously also provide a technique for conducting synchronization sampling in the time domain and averaging in the frequency (order) domain, eliminating the necessity for a high accuracy tachometer (speed) signal. These principles are suitable for bearing monitoring in applications where shaft rotating speed(s) are variable and a physical tachometer is not feasible. While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Patent Citations
Non-Patent Citations
Referenced by
Classifications
Legal Events
Rotate |